用户在登录FTP服务器的过程中，建立TCP连接时使用的默认端口号是（ ）。
FTP服务基于传输层TCP协议，使用21和20端口，其中建立TCP连接使用端口21，数据传输使用端口20。22端口是默认的SSH端口，23端口主要用于Telnet（远程登录）服务。
Unified Modeling Language（UML） is a widely used method of visualizing and documenting an information system. The UML can be used to develop（ 71 ）, in which an object represents a person, place, event, or transaction that is significant to the information system. Systems analysts define an object's attributes during the（ 72 ）. An object also has（ 73 ）which are tasks or functions that the object performs when it receives a message, or command, to do so. A（n）（ 74 ）is a group of similar objects. If objects are similar to nouns, attributes are similar to adjectives that describe the characteristics of an object. Objects can have a specific attribute called a（n）（ 75 ）, which of an object is an adjective that describes the object's current status. All objects within a class share common attributes and methods, so a class is like a blueprint, or template for all the objects within the class.
Unified Modeling Language（UML） is a widely used method of visualizing and documenting an information system. The UML can be used to develop（ 71 ）, in which an object represents a person, place, event, or transaction that is significant to the information system. Systems analysts define an object's attributes during the（ 72 ）. An object also has（ 73 ）which are tasks or functions that the object performs when it receives a message, or command, to do so. A（n）（ 74 ）is a group of similar objects. If objects are similar to nouns, attributes are similar to adjectives that describe the characteristics of an object. Objects can have a specific attribute called a（n）（ 75 ）, which of an object is an adjective that describes the object's current status. All objects within a class share common attributes and methods, so a class is like a blueprint, or template for all the objects within the class.
Unified Modeling Language（UML） is a widely used method of visualizing and documenting an information system. The UML can be used to develop（ 71 ）, in which an object represents a person, place, event, or transaction that is significant to the information system. Systems analysts define an object's attributes during the（ 72 ）. An object also has（ 73 ）which are tasks or functions that the object performs when it receives a message, or command, to do so. A（n）（ 74 ）is a group of similar objects. If objects are similar to nouns, attributes are similar to adjectives that describe the characteristics of an object. Objects can have a specific attribute called a（n）（ 75 ）, which of an object is an adjective that describes the object's current status. All objects within a class share common attributes and methods, so a class is like a blueprint, or template for all the objects within the class.
Unified Modeling Language（UML） is a widely used method of visualizing and documenting an information system. The UML can be used to develop（ 71 ）, in which an object represents a person, place, event, or transaction that is significant to the information system. Systems analysts define an object's attributes during the（ 72 ）. An object also has（ 73 ）which are tasks or functions that the object performs when it receives a message, or command, to do so. A（n）（ 74 ）is a group of similar objects. If objects are similar to nouns, attributes are similar to adjectives that describe the characteristics of an object. Objects can have a specific attribute called a（n）（ 75 ）, which of an object is an adjective that describes the object's current status. All objects within a class share common attributes and methods, so a class is like a blueprint, or template for all the objects within the class.
Unified Modeling Language（UML） is a widely used method of visualizing and documenting an information system. The UML can be used to develop（ 71 ）, in which an object represents a person, place, event, or transaction that is significant to the information system. Systems analysts define an object's attributes during the（ 72 ）. An object also has（ 73 ）which are tasks or functions that the object performs when it receives a message, or command, to do so. A（n）（ 74 ）is a group of similar objects. If objects are similar to nouns, attributes are similar to adjectives that describe the characteristics of an object. Objects can have a specific attribute called a（n）（ 75 ）, which of an object is an adjective that describes the object's current status. All objects within a class share common attributes and methods, so a class is like a blueprint, or template for all the objects within the class.
为了控制IP报文在网络中无限转发，在IPv4数据报首部中设置了（ ）字段。
在Linux系统中，DNS配置文件的（ ）参数，用于确定DNS服务器地址。
用户在登录FTP服务器的过程中，建立TCP连接时使用的默认端口号是（ ）。
使用电子邮件客户端从服务器下载邮件，能实现邮件的移动、删除等操作在客户端和邮箱上更新同步，所使用的电子邮件接收协议是（ ）。
Cookie为客户端持久保持数据提供了方便，但也存在一定的弊端。下列选项中，不属于Cookie弊端的是（ ）。
